An active pixel sensor (APS) combines a photodiode with processing and amplification circuitry in each pixel to form an image sensor. In some implementations, the associated circuitry is implemented with four transistors, and may be referred to as a 4 T APS. Such sensors are commonly found in devices such as cell phone cameras, digital cameras, and web cameras.
Smaller APS and smaller pixels are enabled by the improvements in semiconductor processing. A consequence of smaller pixel size is that the pixel's photodiode must also shrink to accommodate the smaller APS area. Less light is then able to fall on each pixel, and as a result image quality is reduced. Conventionally, as pixel size is getting smaller, a plurality of pixels share transistor(s) so to increase fill factor, which represents the percentage of the pixel area that is consumed by the actual photodiode, and is a higher-is-better (HB) metric.
However, when a plurality of pixels share a transistor, each pixel contributes additional parasitic capacitance to the pixel sensor. The higher capacitance of the pixel sensor results in a lower conversion gain (CG), which is undesirable.
It is desirable to maintain conversion gain when more pixels share electrical circuitry. Further, it is desired to have high CG, especially for high sensitivity under low illumination conditions.